Tropical Storm GAEMI Advisory Mon Oct 01

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE)
WARNING NR 01//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 21W, LOCATED APPROXIMATELY 300 NM SOUTH OF
HONG KONG, HAS TRACKED NORTH-NORTHEASTWARD AT 05 KNOTS OVER THE PAST
SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS A WELL
DEFINED LOW LEVEL CIRCULATION CENTER (LLCC) WITH A DEVELOPING BAND
OF DEEP CONVECTION BUILDING IN THE SOUTHEASTERN QUADRANT AND
WRAPPING INTO THE CENTER OF THE LLCC. THE SYSTEM HAS BEEN QUASI-
STATIONARY OVER THE PAST 12 HOURS, BUT THERE WERE INDICATIONS OF
MULTIPLE WEAKER CIRCULATIONS LOCATED WITHIN THE REGION. THE CURRENT
POSITION IS BASED ON THE AVAILABLE IMAGERY AND IS FOCUSED ON THE
PRIMARY CIRCULATION. THE CURRENT INTENSITY OF 25 KNOTS IS BASED ON
THE DVORAK ESTIMATES FROM PGTW AND AN OLDER 0451Z OSCAT PASS SHOWING
RAIN-FLAGGED 25 TO 30 KNOT WINDS. THE LLCC APPEARS TO BE SLOWLY
DRIFTING NORTHWARD AS IT HAS CONSOLIDATED, BUT IS CURRENTLY IN A
WEAK STEERING ENVIRONMENT. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM
IS LOCATED IN A DIFFLUENT REGION OF AN UPPER LEVEL SUBTROPICAL RIDGE
(STR). SEA SURFACE TEMPERATURES (SSTS) ARE VERY FAVORABLE THROUGHOUT
THE AREA (30 DEGREES CELSIUS).
3. FORECAST REASONING.
   A. THIS IS THE INITIAL PROGNOSTIC REASONING AND SETS THE FORECAST
PHILOSOPHY.
   B. TD 21W IS EXPECTED TO SLOWLY DRIFT NORTHWARD AS A DEEP MID-
LATITUDE TROUGH HAS WEAKENED THE STR THAT HAD BEEN LOCATED OVER
SOUTHEASTERN CHINA. AS THE TROUGH LIFTS AND MOVES EASTWARD, THE STR
WILL BUILD BACK OVER THE NORTHERN PORTION OF THE SOUTH CHINA SEA,
AND WILL START TO TRACK TD 21W WESTWARD BY TAU 36 TO 48. INTENSITY
WILL INCREASE SLOWLY DUE TO FAVORABLE SSTS, AND A STEADY EQUATORWARD
OUTFLOW CHANNEL. THE LIMITING FACTOR APPEARS TO BE THE POLEWARD
OUTFLOW CHANNEL THAT IS HINDERED BY THE NORTHWESTERLY FLOW AROUND
THE EASTERN EXTENT OF THE STR AND DEEP MID-LATITUDE TROUGH CREATING
PRESSURE ALONG THE NORTHERN HALF OF THE SYSTEM.
   C. BEYOND TAU 72, TD 21W IS EXPECTED TO CONTINUE TRACKING
WESTWARD UNDER THE INFLUENCE OF THE STR. INTENSITY WILL STEADILY
BUILD BUT AS PREVIOUSLY MENTIONED WILL BE LIMITED BY THE LACK OF
DUAL OUTFLOW CHANNELS. MODEL GUIDANCE IS IN GENERAL AGREEMENT WITH A
WESTWARD TRACK IN THE EXTENDED TAUS, HOWEVER IN THE EARLY TIMEFRAME
THERE IS A LACK OF CONSISTENCY BETWEEN THE MODELS. THIS FORECAST
FAVORS A SLOW NORTHWARD DRIFT WITH CAPTURE BY THE STR. AS THIS IS
THE FIRST FORECAST, AND THE MODEL GUIDANCE IS IN POOR AGREEMENT,
THERE IS LOW CONFIDENCE IN THE FORECAST TRACK AND INTENSITY
DEVELOPMENT.//
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